It's Complicated: Calories and Other Factors Affect Weight Loss

March 18, 2013

ROCHESTER, Minn. — Reducing calories is key to weight loss, but the relationship between calories and weight loss is complicated. The March issue of Mayo Clinic Health Letter clarifies some confusion around calorie counting and weight loss.

It's commonly accepted that 1 pound of fat equals 3,500 calories. Therefore, a reduction of 500 calories a day — or 3,500 a week — should mean losing a pound of fat. However, metabolism, body composition and even how the brain perceives food and drink can affect the formula.

A pound of fat, muscle and water: When dieters lose a pound, it's not just fat; it's a mixture of fat, lean tissue such as muscle, and water. The diet that is followed also affects the composition and pace of weight loss, independent from calories cut from the diet. For example, a low-carbohydrate diet leads to water loss in the body and quicker weight loss than would be expected based on calorie reduction. A caveat: Water weight loss is often temporary.

Metabolic rate: This is the energy the body burns at rest. Basic organ function accounts for about 70 percent of metabolic rate. Most of the remainder comes from maintenance of lean muscle tissue. Fat burns very few calories at rest. People with lower amounts of muscle tissue may not burn as many calories at rest as someone with more, making weight loss more difficult. Older adults have less muscle and a lower base metabolism. In general, men have more lean tissue than women.

Response to reduced calories: Drastic reduction in calories or eating all daily calories in a single meal sets off a cascade of responses in the body. For example, the body conserves energy by reducing the rate of metabolism, making it hard to lose an amount of weight that corresponds to calorie reduction. A better weight-loss strategy is to reduce calories gradually and spread calories eaten over a few meals a day.

Food and drink choices: Researchers aren't sure why, but calories consumed in moderate amounts of alcohol or moderate portions of nuts don't appear to contribute to weight gain. On the other end of the scale, sugar substitutes found in products such as diet soda have few calories, yet people who drink diet soda tend to weigh more than those who don't.

Other factors: Stress levels, genetics, amount of sleep and bacteria in the gut are other factors that can skew the calorie and weight-loss formula.

While the exact calculation of calories to weight loss is complex, the two basics of weight control and weight loss are simple. It's best to eat a low-calorie diet that's heavy on fruits, vegetables, whole grains and nuts. And, it's important to be active, with a goal of 150 minutes of moderately intense physical activity a week.